This invention relates to reflection of light and vapor deposition of metals.
Silver is a desirable reflector for telescope mirrors used in high power laser applications and in other applications where very high reflectivity is required. Silver is an excellent reflector in the visible and near infrared regions of the spectrum and has a high threshold of resistance to laser damage and radiation-induced damage.
Silver reacts with trace amounts of sulfur compounds in the atmosphere and a sulfide film quickly forms on the surface of an unprotected silver reflector, causing significant degradation of reflectivity. Reflectivity at glancing angles of incidence, where the incident radiation strikes the mirror at angles other than 90.degree. to the mirror surface, is particularly sensitive to this tarnishing. This has severely limited the use of bare silver. Overcoating of a silver surface with various transparent substances has been used to protect the silver, but reflectivity properties of a silver reflector may be changed by the coating and the laser damage and radiation damage thresholds are usually degraded by the coating.
Aluminum oxide, or alumina, has been used to coat silver reflectors to prevent sulfide corrosion by forming a barrier between the silver and the atmosphere, but such coatings have been much thicker than the coating of the present invention and have not been considered by those working in this area to have solved the problem. These thick coatings contain pinholes and decrease reflectivity. Also, a thick alumina coating deteriorates in the presence of radiation such as neutrons and gramma rays; color centers are created in alumina by the radiation. A color center is a defect in a crystal lattice which is occupied by an electron, where the defect of absorbs light, thereby causing the defect to appear as a color other than white. When exposed to laser light, the color centers absorb the light instead of reflecting it. Also, the laser damage threshold of the coating is degraded by the presence of color centers. When the alumina coating is very thin, as in the present invention, the absorption of light by any color centers which form will be minimal.
Following are brief descriptions of patents and an article which may be of interest.
1. George H. Crawford et al., "Metal/Metal Oxide Coating," U.S. Pat. No. 4,430,366, February 1984. This patent teaches a method of deposition, on a substrate, of a layer or layers of certain metals, oxides, or sulfides of these metals, or a metal and its oxide or sulfide in a continuous and rapid manner. Deposition of layers from molecular dimensions to micrometers is mentioned (column 2, line 65-68) and it is then said that preferably the layer is between 50 and 5000 .ANG. thick when used in imaging applications and between 15 and 25 .ANG. thick when used to cause a layer of another substance to bond to it. This patent contains no mention of use of these coatings for the purpose of corrosion protection.
2. G. W. DeBell et al., "Optical Component of a Laser," U.S. Pat. No. 4,685,110, August 1987. This patent teaches that metal oxides and certain other substances may be used to coat optical elements of a laser apparatus, such as windows, in order to prevent physical and chemical changes induced by the large photon fluxes of the laser. The usual change induced by large photon fluxes is photoreduction; for example, crystalline silicon dioxide may be reduced to elemental silicon. This patent mentions coatings over 50 .ANG. thick, half wave optical thickness (absentee layer), and integral multiples of half wave optical thickness.
3. V. Grewal et al., "Mirror Structure," U.S. Pat. No. 4,482,209, November 1984. This patent teaches a mirror having an adhesive layer which may be titanium or chromium, an intermediate layer which may be aluminum oxide, a reflective layer which may be silver, and a protective layer which may be alumina, where the protective layer will have a thickness of about 300 to 3000 .ANG..
4. G. Hass et al., "Reflectance and Durability of Ag Mirrors Coated with Thin Layers of Al.sub.2 O.sub.3 Plus Reactively Deposited Silicon Oxide," Journal of Applied Optics, Vol. 14, No. 11 (1975). This article teaches a combined coating for a silver mirror consisting of a layer of alumina covered by a layer of silicon oxide. It is stated that the optimum thicknesses of the two layers are about 300 .ANG. for the alumina layer and about 1500 .ANG. for the silicon oxide layer.
5. F. I. Brown, "Method of Improving the Conductivity and Lowering the Emissivity of a Doped Tin Oxide Film," U.S Pat. No. 4,721,632, January 1988. This patent is directed to a method of improving the conductivity and lowering the emissivity of a doped tin oxide film on the surface of a soda lime silica glass sheet.
6. A. K. Chin et al., "Process for Making Optical INP Devices," U.S. Pat. No. 4,617,192, October 1986. This patent teaches a process for coating optical surfaces with aluminum oxide using electron-beam deposition in an oxygen enriched atmosphere. The primary purpose of the process is to produce anti-reflecting coatings, though it is stated that the coatings can be used for other purposes. The typical thickness of an alumina coating is 1850 .ANG..
7. H. N. Adams et al., "Protective Coating for Surfaces of Silver and Mirror Fabrication," U.S. Pat. No. 3,687,713, August 1972. This patent teaches a protective coating for silver and other substances consisting of a layer of aluminum oxide covered with a layer of silicon dioxide. The aluminum oxide layer is deposited to a thickness of about 1000 to 1100 .ANG..
8. K. Nishida et al., "Reflecting Mirror," U.S. Pat. No. 4,009,947, March 1977. This patent teaches a mirror comprised of a glass substrate, a silver layer, and protective layer consisting of a copper-tin alloy.